The present invention relates to a magnetic resonance imaging apparatus.
Magnetic resonance imaging (MRI) apparatuses are widely used in various fields including a medical application and an industrial application.
A magnetic resonance imaging apparatus emits an electromagnetic wave toward a subject in a static magnetic field space to thereby excite spins of protons in the subject with a nuclear magnetic resonance (NMR) phenomenon, and conducts a scan to acquire magnetic resonance (MR) signals generated by the excited spins. The magnetic resonance signals acquired in the scan are used as raw data for a slice image to produce a slice image of the subject.
In such a scan on a subject using the magnetic resonance imaging apparatus, if the subject move, motion artifacts may appear in a produced slice image. For example, when the heart or abdomen of the subject is imaged, body motion such as respiratory or cardiac motion leads to development of motion artifacts and degenerates image quality.
To prevent such image quality degeneration due to motion artifacts, there is proposed a method of conducting a scan in synchronization with body motion such as respiratory or cardiac motion (see Patent Document 1, for example).
[Patent Document 1] Japanese Patent Application Laid Open No. H10-277010
[Patent Document 2] Japanese Patent Application Laid Open No. 2002-102201
In such a method, a displacement caused by cyclic cardiac motion is detected as electrocardiographic signals, for example, and the magnetic resonance imaging apparatus repetitively scans the subject at a specific phase of cardiac motion of the subject based on the electrocardiographic signals. In the scan, first, a region containing the diaphragm, for example, is selectively excited to monitor respiratory motion of the subject, and a navigator sequence is performed to acquire magnetic resonance signals as navigator echo data. Subsequent to the navigator sequence, an imaging sequence is performed to acquire magnetic resonance signals as imaging data from a slice position at which a slice image is to be produced. At that time, if a displacement of the diaphragm obtained by the navigator sequence falls within a predefined acceptance window, the imaging data acquired by the subsequent imaging sequence is selected as raw data for the slice image to sequentially fill a k-space. In particular, since the heart rate of the subject is generally of the order of sixty beats per minute, navigator echo data and imaging data are acquired in a cycle of one second, and imaging data that is acquired when a displacement of the diaphragm obtained by the navigator echo data falls within a predefined acceptance window is selected as raw data, which is for use as a material for a slice image. A slice image is then reconstructed based on the imaging data selected as raw data.
However, when respiratory motion becomes arrhythmic, i.e., for example, when the depth of respiration is significantly disturbed and varies, the diaphragm may lie at a position different from that in cyclic respiratory motion in performing the imaging sequence, even though a displacement of the diaphragm obtained by the navigator sequence performed before the imaging sequence falls within the predefined acceptance window. In such a case, motion artifacts may appear, thus leading to degeneration of image quality.
As described above, according to the conventional techniques, development of motion artifacts cannot be sufficiently suppressed and it is difficult to improve image quality.